This invention relates to a mirror projection system for use in a step-and-scan lithographic projection apparatus for imaging a mask pattern, present in a mask, on a substrate by means of a beam of EUV radiation, which beam has a circular segment-shaped cross-section, said projection system being constituted by six imaging mirrors having ordinal numbers 1-6 from the object side to the image side, the first, the second, the fourth and the sixth mirror being concave and the fifth mirror being convex.
The invention also relates to a lithographic apparatus for step-and-scan imaging of a mask pattern on a number of areas of a substrate, which apparatus comprises such a mirror projection system.
EP-A 0 779 528 describes a mirror projection system for use in a step-and-scan lithographic apparatus with which an IC mask pattern is imaged on a number of areas of a semiconductor substrate, using EUV radiation. EUV, extreme ultraviolet, radiation is understood to mean radiation having a wavelength in the range between several nm and several tens of nm. This radiation is also referred to as soft X-ray radiation. The use of EUV radiation provides the great advantage that extremely small details, of the order of 0.1 .mu.m or less, can be imaged satisfactorily. In other words, an imaging system in which EUV radiation is used has a very high resolving power without the NA of the system having to be extremely large, so that also the depth of focus of the system still has a reasonably large value. Since no suitable material of which lenses can be made is available for EUV radiation, a mirror projection system must be used for imaging the mask pattern on the substrate, instead of a hitherto conventional lens projection system.
The lithographic apparatuses currently used in the production of ICs are stepping apparatuses. In these apparatuses, a full field illumination is used, i.e. all areas of the mask pattern are illuminated simultaneously and these areas are simultaneously imaged on one IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area will be positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate of the mask pattern are illuminated. As is known, it remains desirable to have ICs with an increasing number of components.
It is attempted to meet this desire not only by reducing the dimensions of these components but also by enlarging the surface areas of the ICs. This means that the, already relatively high, NA of the projection lens system must be further increased and, for a stepping apparatus, the image field of this system must also be further increased. This is practically impossible.
It has therefore been proposed to change from a stepping apparatus to a step-and-scan apparatus. In such an apparatus, a rectangular or circular segment-shaped sub-area of the mask pattern and hence also such a sub-area of an IC area of the substrate is illuminated, and the mask pattern and the substrate are moved synchronously through the illumination beam, taking the magnification of the projection system into account. A subsequent circular segment-shaped sub-area of the mask pattern is then imaged each time on a corresponding sub-area of the relevant IC area on the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of a subsequent IC area is introduced into the projection beam and the mask is set to its initial position, whereafter said subsequent IC area is scan-illuminated via the mask pattern. This scan-imaging method may be used to great advantage in a lithographic apparatus in which EUV radiation is used as the projection radiation.
The embodiment of the projection system described in EP 0 779 528, intended for use with EUV radiation having a wavelength of 13 nm, has an NA of 0.20 at the image side. The annular image field has an inner radius of 29 mm and an outer radius of 31 mm and a length of 30 mm. The resolution of the system is 50 nm and the aberrations and distortions are sufficiently small to form a good image of a transmission mask pattern on an IC area of a substrate by way of a scanning process. The third mirror of this projection system is concave. A first pair of mirrors, consisting of the first and the second mirror, constitutes a magnified image of the object or the mask pattern. This image is transported by a second pair of mirrors, constituted by the third and the fourth mirror, and presented to a third pair of mirrors, constituted by the fifth and the sixth mirror, which provides the desired telecentric image with the required aperture NA=0.20. In this projection system, an intermediate image is formed between the third and the fourth mirror, and a diaphragm is situated on the second mirror.
In the known projection system, the mirror sections which constitute the third and the fourth mirror must be located at a relatively large distance from the optical axis of the system. This may cause alignment and stability problems. Moreover, the known system has a small free working distance of the order of 17 mm. In practice, a larger working distance is often required, for example, in connection with building in measuring systems.
It is an object of the present invention to provide a projection system of the type described in the opening paragraph, having a relatively large free working distance and being stable. To this end, the projection system according to the invention is characterized in that the third mirror is convex.